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Why A Mysterious 6th Sense Makes Us Complete

Updated: Feb 7

What 19th-century neuroanatomist Sir Charles Bell called the sixth sense is probably one you didn’t know you have.

In 1971, 19-year-old Ian Waterman awoke and discovered his body had disappeared. He could see his physical body, yet he was unable to move or feel it. Immobilized by a rare neurological illness caused by a rare autoimmune response, his immune system attacked the sensory neurons from his neck down. He was unable to feel any sensation below the neck, yet he was not paralyzed in the traditional sense. When his mother tried to help him stand, he collapsed to the floor because he could not control his limbs in gravity. Waterman lost the ability to sense proprioception, or where the body is in space. It is this “sixth sense” that many take for granted, and one that most don’t know they have.

Mind Over Matter

Waterman lost the basic ability to sense himself and the presence of his own body, and he began to feel that he did not exist. The information that Waterman lost is a complex system of neural pathways that subsist throughout the body, specifically in the joints, tendons, and muscles. Standing, balancing, performing coordinated movements, and maneuvering through crowds are all done using proprioception.

Refusing to live life in a wheelchair, Waterman, once a butcher by trade, where intricate movements were part of his skill, began to visualize his movements as though he were performing them. One day while lying on his back, he imagined the physical act of sitting up. He began to do this daily and started to see small movements, even though he still felt nothing. Finally, by using intent for the physical action, he was able to sit up.

Waterman taught himself how to regain his movements again by consciously controlling and visually tracking every action. The visual component was vital, and if the lights went out suddenly, he collapsed to the floor as though he were unplugged. If the lights were restored, he could move again and stand. Dr. Jonathan Cole, who practices at Poole Hospital, England, and Southampton University, was the first physician to recognize what Waterman had begun to do in his self-training protocol. He wrote about it in the book Pride and a Daily Marathon, which tells Waterman’s incredible story. “Before I met Jonathan, I often thought I might be mad,” says Waterman. “No one understood what was wrong or why life was such a struggle.”

The Importance of Proprioception

Waterman’s experience resonated in the medical field, as physicians were fascinated with Waterman’s ability to compensate for his loss. While others with the same condition often refused to learn to walk again, Waterman did not surrender. Yet his case also emphasizes the critical importance of proprioception and touch, according to psychologist Michael Turvey, Ph.D., who researches touch at the University of Connecticut. “The haptic senses underlie almost everything we do that involves movement,” he says. “At the same time, Waterman can do more than many theories of touch and movement would predict.” The case also presents a unique moment to test theories of movement and proprioception that would not be possible otherwise. Never have researchers been able to examine precisely how a complete lack of feedback from the outside world affects a living human.

Physiologists of the 19th century did not know specialized proprioceptors existed. They questioned the origin of ‘muscle sense,’ a term credited to the Scottish physician, scientist, and neural system researcher physiologist Charles Bell, who first distinguished motor from sensory nerves. Proprioception in the body is governed by mechanosensory neurons distributed throughout the body, also known as proprioceptors. This wasn’t always known; early physiologists assumed all awareness of body position is central and controlled by the brain only. In the late 19th century, neurophysiologist Charles Sherrington proved that a peripheral source of sensory receptors was located in muscles and tendons and that the nervous system, including the brain, is a single interlinking network.

Sherrington created the term proprioception, which he defined as the feeling of stimuli that “are traceable to actions of the organism itself.” His sensory theory was based on what he called exteroception or stimuli existing outside the body, and interception, the sensory signals from the internal organs. While other physiologists like Kuhne and Ruffini had defined proprioceptor organs, Sherrington was the first to apply sensory neurons' influence on innervating organs and muscles for posture and movement control.

Today, the study of sensorimotor control still centers around Sherrington’s initial model of the proprioceptive system, especially his focus on ‘reflex’ pathways that guide proprioceptive feedback into motor output. How do these receptors help suspend us in gravity? The body has three distinct types of sensors located in the joints, in the connections between muscles, and in the muscle fibers. Without them, after hip surgery for example, a patient would not be able to sense where her feet were in relation to the hip. This would make physical therapy impossible.

How He Got His Body Back

A muscle spindle includes four different nerve endings that spiral around several muscle fibers. The small sensor, only a centimeter long, is enveloped in cone-shaped fascia that divides it from the rest of the muscle. The proprioceptors connect the neuron’s varied signals with changes to a muscle. They also remember the position they were in previously when a muscle is motionless. For Waterman, all of these connections were lost. He had to think of movements and have vision to monitor their actions. His case was unique in that he was the first patient to lose his body and teach himself how to walk again. He relied only on the sense of sight and conscious orders to make his legs move.

Waterman has perfected his strategy to an astonishing degree. Since his movements require constant visual contact with the environment and space, he must “plan” his walks. Every movement is choreographed previously, including calculations of the force and direction, and the movements are monitored as he ambulates. He calls his slightly lumbering gait “controlled falling.” While most of his gestures are planned and monitored, research conducted by The University of Chicago’s David McNeill demonstrated some are not. McNeill’s research on Waterman showed the human brain orchestrates communicative motions in the absence of proprioception.

What McNeill discovered through experiments in voice and hand movement is that Waterman uses a thought-language-hand link to modulate some of his hand gestures. His gestures modulated with the speed of his speech. If he spoke slowly, his gestures slowed too. Yet the speech-gesture synchrony was exactly preserved. Waterman’s case suggests that control of the hands and the relative motor neurons is possible directly from the thought-linguistic system. While Waterman had reworked his motion and control system, the thought-language-hand link system in the human brain survived his neuronopathy.

Despite his disability, Ian Waterman has taken his life very seriously, always attempting to do the best he can. While he doesn’t care about the science of his illness, he has helped change it with hours of lab experiments. “We choose our own paths,” he says.

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